Release coating compositions are used, for example, to coat papers or films which are used as release backing sheets (sometimes referred to as protective release sheets or release liners), overlying the pressure sensitive adhesive used in pressure-sensitive products such as labels, decals, tapes, etc. The pressure sensitive adhesive adheres to the release coated surface of the release backing sheet sufficiently to enable the pressure sensitive products to be handled prior to use. When such products are used, the release backing sheet is pulled off and discarded. The exposed pressure-sensitive adhesive is pressed onto a surface where the pressure sensitive product is to be placed. Known types of release backing sheets include paper release backing sheets wherein one side of the paper (the release side) is coated with a silicone polymer or copolymer.
Silicone polymers and copolymers have been used extensively in providing release coatings on paper, film, etc., because they are inherently low in surface energy. It is desirable that release-coated papers and films have a release force which is low enough to enable the release backing sheet to be easily removed from a pressure sensitive adhesive coated substrate, but not so low that the release backing sheet will become separated from the pressure sensitive adhesive prior to when desired by forces normally encountered in handling and processing such as printing, die cutting and matrix stripping. "Release force" is defined as the amount of force required to peel or separate the release-coated substrate from the adhesive.
The silicone polymers and copolymers (sometimes referred to as polyorganosiloxanes) used in the prior art for making release coating compositions can be radiation cured or thermally cured. The radiation cured polymers have a number of advantages over the thermally cured polymers including better adhesion to certain substrates, higher cure processing speeds, and lower high speed release. However, the radiation cured polymers are often more brittle than the thermally cured polymers and thus problems often occur during handling and processing, especially during die cutting. It would be advantageous to provide a release coating composition that utilized such radiation cured polymers and yet did not exhibit the brittleness problems typically incurred with such polymers.
Polyorganosiloxanes containing functional groups which can be radiation cured in the presence or absence of a photosensitizer have been described in various patents as useful silicone release compositions. U.S. Pat. No. 3,726,710 describes radiation-curable release compositions comprising an polyorganosiloxane having olefinically unsaturated organic groups and a photosensitizer. The composition may be applied to a paper substrate and cured by exposure to high intensity radiation to produce a release coating.
The use of polyorganosiloxanes with acrylate or methacrylate ester groups linked to SiC groups as radiation-curable coating materials is described in, for example, U.S. Pat. Nos. 3,878,263; 4,064,286; 4,963,438; 4,908,274; 4,978,726; and 5,034,491.
The use of epoxypolyorganosiloxanes in making release coating compositions is described in U.S. Pat. Nos. 4,279,717; 4,313,988; 4,421,904; 4,547,431; 4,822,687; 5,217,805; 5,279,860; 5,332,797; and 5,360,833.
It is well known to use cellulose fibers as fillers in plastics. This use is disclosed, for example, in Katz et al., "Handbook of Fillers and Reinforcements for Plastics", Van Nostrand Reinhold Co., 1978, pages 292-294.
U.S. Pat. Nos. 3,503,782 discloses a differential release paper having a initial coating of a first release agent and a coating of a second release agent applied over a portion of the initial coat, each of the release agents having different release characteristics. The reference indicates that filler material in finely divided form can be added to the release agent coatings to control rheological properties and release characteristics. The filler materials are identified as calcium carbonate, titanium dioxide, silica, alumina, mica, calcium silicate, zinc oxide, iron oxide, polyvinylchloride, polyesters, polyamides and polycarbonates.